g._ the not-great is as truly real as the great, then every conception may be apprehended according to its opposite sides as being and not-being at the same time: it is a being in reference to itself, as something identical with itself, but it is not-being in reference to every one of the numberless other conceptions which can be referred to it, and with which, on account of its difference from them, it can have nothing in common. The conception of the same ταὐτὸν and the different θάτερον represent the general form of an antithesis. These are the universal formulæ of combination for all conceptions. This reciprocal relation of conceptions as at the same time being and not-being, by virtue of which they can be arranged among themselves, forms now the basis for the art of dialectics, which has to judge what conceptions can and what cannot be joined together. Plato illustrates here by taking the conceptions of being, motion (becoming), and rest (existence), and showing what are the results of the combinations of these ideas. The conceptions of motion and rest cannot well be joined together, though both of them may be joined with that of being, since both are; the conception of rest is therefore in reference to itself a being, but in reference to the conception of motion a not-being or different. Thus the Platonic doctrine of ideas, after having in the Theatætus attained its general foundation in fixing the objective reality of conceptions, becomes now still farther developed in the Sophist to a doctrine of the agreement and disagreement of conceptions. The category which conditions these reciprocal relations is that of not-being or difference. This fundamental thought of the Sophist, that being is not without not-being and not-being is not without being, may be expressed in modern phraseology thus: negation is not not-being but determinateness, and on the other hand all determinateness and concreteness of conceptions, or every thing affirmative can be only through negation; in other words the conception of contradiction is the soul of a philosophical method.